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Methodology The CLMN employs transparency measurement using a Secchi disk.
What is a Secchi disk?
The Secchi disk is the basic tool of lake water quality monitors
worldwide. A Secchi disk is simply a circular metal plate attached to a calibrated rope.
It is probably the least expensive and easiest to use tool in water quality monitoring.
One of the advantages of using a Secchi disk is that the information provided by the
Secchi disk is easily interpreted by volunteers and can be used to detect water quality
trends in lakes. |
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The Secchi disk is an 8-inch (20 cm) in
diameter metal with alternating black and white quadrants. It is lowered into the water of
a lake until it can be no longer seen by the observer. This depth of disappearance, called
the Secchi depth, is a measure of the transparency of the water. The Secchi disk measures
the transparency of the water.
The disk is weighted to ensure that it goes straight down. A calibrated line is attached to the Secchi disk in order to measure the maximum depth at which it is visible. The preferred cord material is Dacron or other polyester material. Although most programs use nylon line, it is not the preferred material here since it has the tendency to stretch when wet as well as gets adversely affected by repeated exposure to sunlight. The line should be calibrated periodically to make sure the markings haven't changed due to shrinkage or stretching. |
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What is Transparency?
Water transparency is a quick and easy measurement that tell scientists important information about the lake's water quality such as:
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Transparency can be also be affected by the color of the water, algae, and suspended
sediments. Transparency decreases as color, suspended sediments, or algal abundance
increases. For example, water is often stained yellow or brown by decaying plant matter.
Algae are small, green aquatic plants whose abundance is related to the amount of
nutrients in the water, especially phosphorus and nitrogen. Thus, transparency can be
affected by the amount of plant nutrients coming into the lake from sources such as sewage
treatment plants, septic tanks, and lawn and agricultural fertilizer. Suspended sediments,
on the other hand, often come from sources such as re-suspension from the lake bottom,
construction sites, agricultural fields, and urban storm runoff. Transparency is an indicator of the impact of human activity on the land area surrounding the lake. If transparency is measured through the season and from year to year, trends in transparency may be observed. Transparency can serve as an early warning that activities on the land are having an effect on a lake.
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